Some subject matter is disclosed and claimed in the following commonly owned, copending U.S. Patent Application which is incorporated herein by reference: xe2x80x9cDevice for Wavefront Correction in an Ultra High Power Laserxe2x80x9d by Earl R. Ault, Brian J. Comaskey and Thomas C. Kuklo.
1. Field of Endeavor
The present invention relates to a laser system that provides high power and high beam quality and particularly to a method for beam steering compensation in an ultra-high power liquid host laser
2. State of Technology
There are many applications for ultra high power lasers. These applications include power beaming, laser guide stars, illuminators, material processing, and weapons. The type of laser incorporating the present invention utilizes a flowing lasing liquid that is optically excited to provide a powerful laser beam. Since the host is a liquid, it can be removed from the optical cavity when it becomes heated avoiding the inevitable optical distortion and birefringence common to glass and crystal hosts.
The heat in the flowing lasing liquid causes a small, but significant, change in the local index of refraction. As the lasing liquid flows, it integrates waste heat and exits the flow region at a slightly higher temperature. This difference is less than a degree centigrade but changes the optical path by many wavelengths of light. For uniform deposition of pump light, the change in index is linear causing a simple optical wedge to accumulate in the optical path.
A liquid laser is described in U.S. Pat. No. 3,717,825 to Carl Zeiss-Stiftung, Wuerttemburg, Federal Republic of Germany, patented Feb. 20, 1973. This patent includes the following description: xe2x80x9c . . . a dyestuff laser provided with a liquid guiding chamber through which circulates a cooled laser liquid. The laser is provided with a U-shaped laser active zone formed by a light transmitting longitudinal cap into which extends a tongue forming in said cap a U-shaped zone of uniform cross section. This U-shaped laser active zone is disposed in a focal line of an elliptically shaped pump light reflector while a source of pump light is disposed in the other focal line.xe2x80x9d
U.S. Pat. No. 3,931,594 to Fritz Peter Schafer, assigned to Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V., patented Jan. 6, 1976, describes a transverse-flow cell for a laser. This patent includes the following description: xe2x80x9cThe cell of a liquid laser is defined between the rounded, parallel edges four coaxial cylinder sectors of fused quartz. The narrow gaps circumferentially bounded by the flat, converging side faces of the sectors provide conduits for rapid flow of dye liquid into and out of the cell which extend over the full axial length of the cell. The light of a flash tube is directed toward the cell through the cylindrically arcuate outer face of each sector.xe2x80x9d
U.S. Pat. No. 3,678,410 to Robert C. Kocher, Franklin K. Moore, Harold Samelson, and William R. Watson, assigned to GTE Laboratories Incorporated, patented Jul. 18, 1972, describes a transverse flowing liquid laser. This patent includes the following description: xe2x80x9cA laser cell for a transverse flow liquid laser has an active region in the form of a rectangular prism and cylindrical input and output chambers mounted in spaced-apart relationship in the transverse direction at opposite ends of the active region. A baffle positioned in the input chamber causes the liquid to flow uniformly through the active region.xe2x80x9d
U.S. Pat. No. 5,189,681 to Paul Blaszuk, assigned to United Technologies Corporation, patented Feb. 23, 1993, describes a high powered laser with reduced optical aberration. This patent includes the following description: xe2x80x9ca laser includes an optical cavity with opposed electrical excitation electrodes adjacent to the optical cavity extending along a portion of an optic axis. A gaseous gain medium is located in the optical cavity and has a first region at a first temperature and a second region adjacent to one of the electrodes at a second temperature. Also included is an unstable optical resonator assembly positioned within the optical cavity that has a first mirror positioned at a first end of the optical cavity that has a focus therein located along the optic axis. A second mirror is positioned opposed to the first mirror at a second end of the optical cavity. The second mirror has a focus within the optical cavity substantially at the first mirror focus. The first and second mirrors are configured to invert an optical beam transiting therebetween, thereby having the optical beam substantially avoid the second gaseous gain medium region.xe2x80x9d
U.S. Pat. No. 4,707,836 to Alan J. B. Travis, assigned to United Kingdom Atomic Energy Authority, patented Nov. 17, 1987, describes a laser control system. This patent includes the following description: xe2x80x9cFor monitoring and controlling a laser beam, sample beams are monitored for three characteristics and the three monitor outputs are used to control the beam generation. The three characteristics monitored are the transverse distribution of energy within the beam (a) under far field conditions and (b) under near field conditions, and (c) total power within a known proportion of the beam. These are employed respectively to control (a) the position of a mirror which forms part of an optical cavity within the laser structure, (b) a beam pointing element, (c) a beam expander and (d) a power supply for the laser.xe2x80x9d
Ultra high power lasers suffer from thermally induced distortion of the optical wavefront caused by heating of the laser media by waste heat from the excitation process and absorption of laser radiation. As the laser medium flows past the optical excitation source and the fluid warms, its index of refraction changes (positive or negative changes) creating an optical wedge. Thermally induced distortion of the optical wavefront caused by heating of the laser media by waste heat from the excitation process and absorption of laser radiation are corrected using the system of the present invention.
The present invention provides a laser system having an optical cavity. A lasing liquid is circulated through the optical cavity. A pumping device within the optical cavity includes power supplies driving semiconductor diodes that excite the lasing liquid. The laser pumping device produces thermally induced optical phase errors. A system is provided for correcting the thermally induced optical phase errors. The system generates an error signal derived from the optical phase errors. A system uses the error signal to feed back to the power supplies driving the semiconductor diodes that excite the lasing liquid to introduce an electrically controllable wedge into the optical cavity to compensate for the thermally induced optical phase errors.
Additional aspects, advantages, and features of the invention are set forth in part in the following description. Various aspects, advantages, and features of the invention will become apparent to those skilled in the art upon examination of the description and by practice of the invention.